1. Field of the Invention
The present invention relates to an optical transmission system, an optical transmitter, and an optical receiver. More particularly, the present invention relates to an optical transmission system in which data is multilevel-coded and the multilevel-coded data is transmitted, an optical transmitter and an optical receiver which the system comprises.
2. Description of the Background Art
In a field of optical communications, as a technique of increasing a transmission speed, TDM (time division multiplexing) and WDM (wavelength division multiplexing) have generally been known.
In addition, as a technique of enhancing a frequency utilization efficiency and of realizing an increase in a transmission speed, multilevel optical transmission in which an amplitude or a phase are multilevel-coded and the multilevel-coded signals are transmitted has been known. In particular, a method of assigning a multilevel code to an amplitude level of a transmitted signal by modulating intensity of light emitted from a light source is, as compared to a method of assigning a multilevel code to a phase, advantageous in that configurations of a transmitter and a receiver can be simplified.
Conventionally, as the method of assigning a multilevel code to an amplitude, a method of using a single light emitting element (for example, refer to Japanese Laid-Open Patent Publication No. 8-79186) and a method of using a plurality of light emitting elements (for example, refer to Japanese Laid-Open Patent Publication No. 2004-112235) have been known. Hereinafter, the both methods will be described.
FIG. 11 is a diagram explaining a multilevel optical signal transmission method in which a single light emitting element is used.
In FIG. 11, a transmitter and receiver circuit 760 comprises a D/A converter circuit 700, a light emitting element 710, an optical fiber 720, a photo-detecting element 730, an amplifier 740, and an A/D converter circuit 750.
First, on a transmitting end, the D/A converter circuit 700 converts a combination of digital signals inputted from a plurality of channels to multi-valued logic levels and generates an analog signal in accordance with the converted logic levels. The light emitting element 710 receives the analog signal outputted from the D/A converter circuit 700 and electric-optic-converts the received analog signal. The light emitting element 710 outputs to the optical fiber 720 an optical signal obtained through the electric-optic conversion.
Next, on a receiving end, the photo-detecting element 730 receives the optical signal outputted from the optical fiber 720 and generates an electrical signal in accordance with intensity of the received optical signal. The amplifier 740 amplifies the electrical signal outputted from the photo-detecting element 730 and outputs the amplified electrical signal to the A/D converter circuit 750. The A/D converter circuit 750 A/D-converts the electrical signal outputted from the amplifier 740 to a digital signal and restores a plurality of digital signals.
FIG. 12 is a diagram explaining a method of transmitting a multilevel optical signal, in which a plurality of light emitting elements are used.
In FIG. 12, an optical transmitter 860 comprises a driver circuit 800 which receives binary signals from four channels and light emitting elements 811 to 814 which are assigned to the four channels.
A driver circuit 800 drives the light emitting elements 811 to 814 in a separate manner so that the light emitting elements 811 to 814 light up or light out respectively. And the driver circuit 800 drives the light emitting elements 811 to 814 whose levels of light-output power level vary respectively. Accordingly, the optical transmitter 860 is capable of outputting 16 kinds of multilevel optical signals, each of which has varying light intensity in accordance with 16 kinds of combinations of lighting-on and lighting-out of the light emitting elements 811 to 814.
However, the above-mentioned multilevel optical signal transmission methods have the following problems, respectively.
Firstly, when the single light emitting element is used for generating multilevel optical signals, because a number of multilevels which can be set in reality is limited, increasing a transmission speed has limitation. Hereinafter, a designing method of multilevels will be described in detail.
FIG. 13 is a diagram showing a current-light intensity characteristic of a semiconductor laser.
As shown in FIG. 13, a curve indicating intensity of light outputted from the semiconductor laser is linear in a region A where an amount of supplied current is equal to or less than a given value, whereas a curve indicating intensity of light outputted from the semiconductor laser is non-linear in a region B where an amount of supplied current exceeds the given value. A reason why the curve indicating the intensity of light outputted from the semiconductor laser is non-linear in the region B is that the output from the semiconductor laser is saturated.
Therefore, as a first condition, it is required that in order to vary the intensity of the optical signals in accordance with a change in a waveform of each of electrical signals, a plurality of light intensity levels P0 to P4 are set to be less than P5 which is a maximum of light intensity in the region A.
Further, as a second condition, a characteristic of an optical receiver is required to be considered. In general, noise generated in the optical receiver increases as intensity of an optical signal received by the optical receiver increases. Therefore, it is favorable that in order to reduce influence of the noise on the optical signal and facilitate identification of the optical signal levels, levels are designed so that a difference between two adjacent levels increases as the intensity of the optical signal increases. For example, as shown in FIG. 13, the levels are designed so that the intensity of outputted light P0 to P5 satisfy a relationship shown by the following inequality.P5−P4>P4−P3> . . . >P1−P0
However, even if the levels are designed so as to satisfy the above-mentioned first and second conditions, in a range of the light intensity of which curve is linear (in the region A in FIG. 13), a plurality of multilevels concentrate in a range in which the intensity of outputted light is low.
When the light emitting element is a semiconductor laser or an LED, a response bandwidth is narrowed as a bias current supplied to the light emitting element decreases. Therefore, when the light emitting element is driven with a low bias current, a response bandwidth of a light source is insufficient as compared to a modulation bandwidth of an inputted signal. As a result, interference among multilevels, which is caused by waveform distortion (closure of an eye pattern), accrues, leading to deterioration in transmission quality.
Therefore, when the multilevel optical signals are generated by a single light emitting element, a number of levels which can be set in a range in which a curve of the intensity of outputted light is linear is limited in reality.
Secondly, when a plurality of light emitting elements are used for generating multilevel optical signals, there accrues a problem of deterioration in transmission quality, which is caused by common mode noise.
Specifically, for example, in the optical transmitter shown in FIG. 12, if noise is generated, the generated noise is likely to be contained as common mode noise in optical signals outputted from the respective light emitting elements. On the other hand, the optical receiver receives the optical signals outputted the respective light emitting elements in a collective manner and outputs electrical signals in accordance with intensity of the received optical signals. As a result, because the optical receiver photoelectric-converts the optical signals in which common mode noise contained in light outputted from the respective light emitting elements are added, influence of the common mode noise increases, leading to deterioration in transmission quality.